It is known that calibration of a display (in medical imaging also called a soft-copy viewing station) is an important component of effective medical imaging (including imaging of anatomy, imaging for diagnostic or clinical use, etc.). In many cases, there are very small luminance or colour differences between an area of interest (which itself may be very small) and the surrounding area. Without proper display system calibration, it is possible that the viewing station itself can adversely affect the ability to make a proper diagnosis or interpretation of the image being displayed. Particularly when using an un-calibrated commercial colour monitor, the low-level shades of grey may be hard to distinguish from one another.
For medical images there have been several guidelines that have been developed for calibration. When the American College of Radiology (ACR) and National Electrical Manufacturers Association (NEMA) formed a joint committee to develop a Standard for Digital Imaging and Communications in Medicine (DICOM), they reserved Part 14 for the Grayscale Standard Display Function (GSDF). This standard defines a way to take the existing Characteristic Curve of a display system (i.e. the Luminance Output in function of each Digital Driving Level DDL or pixel value) and modify it to the Grayscale Standard Display Function. At the heart of the Grayscale Standard Display Function is the Barten Model. This model takes into account the perceptivity of the human eye. Given the black and white levels of the display system, it will spread out the luminance at each of the intermediary Digital Driving Levels such as to maximize the Just Noticeable Differences (JND) between each level. A JND is the luminance difference that a standard human observer can just perceive. Calibration has the aim that each DDL will be as distinguishable as possible from neighbouring levels, throughout the luminance range, and it will be consistent with other display systems that are similarly calibrated.
A part of DICOM, supplement 28, describes the GSDF in more detail (available at http://medical.nema.org/dicom/final/sup28_ft.pdf). It is a formula based on human perception of luminance and is also published as a table (going up to 4000 cd/m2). It also uses linear perceptions and JND. Steps to reach this GSDF on a medical display are named ‘Characterization’, ‘Calibration’ and afterwards a ‘Conformance check’. These will be discussed in more detail below.
FIG. 8 and FIG. 9 are extracts from the document “DICOM/NEMA supplement 28 greyscale standard display function”. FIG. 8 shows the principle of changing the global transfer curve of a display system to obtain a standardised display system 102 according to a standardised greyscale standard display function. In other words, the input-values 104, referred to as P-values 104, are converted by means of a “P-values to DDLs” conversion curve 106 to digital driving values or levels 108, referred to as DDL 108, in such a way that, after a subsequent “DDLs to luminance” conversion, the resulting curve “luminance versus P-values” 114 follows a specific standardised curve. The digital driving levels then are converted by a “DDLs to luminance” conversion curve 110 specific to the display system (native transfer curve of the display system) and thus allow a certain luminance output 112. This standardised luminance output curve is shown in FIG. 9, which is a combination of the “P-values to DDLs” conversion curve 106 and the “DDLs to luminance” curve 110. This curve is based on the human contrast sensitivity as described by the Barten's model. It is to be noted that it is clearly non-linear within the luminance range of medical displays. The greyscale standard display function is defined for the luminance range 0.05 cd/m2 up to 4000 cd/m2. The horizontal axis of FIG. 2 shows the index of the just noticeable differences, referred to as luminance JND, and the vertical axis shows the corresponding luminance values. A luminance JND represents the smallest variation in luminance value that can be perceived at a specific luminance level. A more detailed description can be found in “DICOM/NEMA supplement 28 greyscale standard display function”, published by National Electrical Manufacturers Association in 1998.
A display system that is perfectly calibrated based on the DICOM greyscale standard display function will translate its P-values 104 into luminance values (cd/m2) 112 that are located on the greyscale standard display function (GSDF) and there will be an equal distance in luminance JND-indices between the individual luminance values 112 corresponding with P-values 104. This means that the display system will be perceptually linear: equal differences in P-values 104 will result in the same level of perceptibility at all digital driving-levels 108. In practice the calibration will not be perfect because, typically, only a discrete number of output luminance values (for instance 1024 specific greyscales) are available on the display system. Deviations from the exact GSDF, e.g. up to 10%, are typically considered to be acceptable.
Currently the above steps are done in most cases with quantitative methods by using a measurement device. In that case the accuracy of the GSDF Conformance Check result depends on all kinds of factors like deficiencies of the different devices used. This is not important in this context; running a calibration sequence on a stable, perfectly performing display by using a perfect measurement device, will result in a nearly 100% match on the GSDF (there still is a quantisation error present and also some instability over time, temperature, . . . ). On the other hand, solutions are known to reach the DICOM GSDF without using a measurement device, but by using a visual procedure.
Known calibration tools include visual test patterns and a handheld luminance meter (sometimes referred to as a “puck”) or a built-in sensor, to measure the conformance to the DICOM standard. These can provide the data to generate a custom LUT correction for DICOM Grayscale Display Function compliance. It is known to provide calibration software, such as the CFS™ (Calibration Feedback System) obtainable from Image Systems Corporation, Minnetonka, Minn., USA, to schedule when a conformance check occurs, and to generate a new DICOM correction LUT if needed. A log of tests and activity can provide a verifiable record of compliance testing, and reduce the need for technicians to take manual measurements.
Both CRT-based and LCD-based display monitors have been successfully used in medical imaging applications. From a calibration standpoint, a LCD-based display is typically more stable when viewed on-axis than a CRT-based display. A CRT can have variations from the electron gun, phosphor, and power supply that will disturb brightness settings and calibration. The LCD's primary source of variation is the backlight, although temperature, ambient lighting changes, and shock/vibration will also have effects. The characteristic curve of an un-calibrated LCD is poor in the sense of DICOM conformance, especially in the low-level grey shade regions. It is known to implement an initial DICOM correction (typically done via a Look-Up Table or LUT), before utilizing the display for diagnosis, and then make periodic measurements to ensure that the calibration correction is still accurate. Liability concerns mean that institutions need to show that they have properly implemented calibration into their medical imaging process. This involves the documentation of objective evidence that the viewing stations have been properly calibrated.
However, a major disadvantage of LCD monitors is that their behaviour (both as described with luminance and colour point) changes significantly when viewed off-axis. Several solutions exist to solve this problem. A first possible solution is to add compensation foils to the optical stack of the LCD. These compensation foils have shown to significantly improve the viewing angle behaviour of twisted nematic, VA (vertical alignment) and IPS (in-plane switching) LCDs. However, LCDs with compensation foils still show an undesirable off-axis viewing behaviour especially for particular critical applications such as medical imaging.
A second possible solution is adding a head-tracking system to the display. This head tracking system determines the position of the user and therefore the current viewing angle under which the user looks at the display. Once the viewing angle is known then it is easy to adapt the transfer curve (luminance and or colour) of the display to compensate for the off-axis viewing behaviour of the display. Such a technique is described for instance in the conference proceedings of SID 2004: “Adaptive Display Color Correction based on real-time Viewing Angle Estimation” by Baoxin Li et al. It is however a disadvantage of this technique that expensive extra hardware is required (a head-tracking system). Another disadvantage of this technique is that still the display behaviour is only correct for one particular angle and therefore the accuracy of the head tracking system determines the display performance. Moreover, in case of multiple viewers therefore this is not a suitable solution as the display behaviour can in general only be set correctly for one user.
A third possible solution to overcome the poor viewing-angle behaviour, of LCD is described in the conference proceedings of SID 2002: “Low-cost Method to Improve Viewing-Angle Characteristics of Twisted-Nematic Mode Liquid-Crystal Displays” by S. L. Wright et al. This solution uses a dithering technique to obtain better off-axis image quality. This technique is based on the idea of replacing grey levels with poor off-axis image quality by a combination of two or more grey levels with better off-axis image quality. The combination of those two or more grey levels results in (approximately) the same luminance value and/or colour point as the original grey level. A major disadvantage of this technique is that the effective resolution of the display is seriously decreased. Indeed: if a 2×2 dither block is used then the effective resolution is only one fourth of the original resolution. In case of LCDs with special pixel structure like monochrome medical LCDs having three grey sub pixels one could avoid this loss of resolution. In this situation it is possible to create a “3×1” dither block consisting of the three sub pixels of one LCD pixel. However, in case of normal pixel structures and especially with colour LCDs this loss of resolution cannot be overcome. An additional disadvantage of the technique described by S. L. Wright is that extra high-frequency noise is added in the image. Indeed: one grey level is replaced by multiple grey levels with possibly large differences between them. In the NPS (noise power spectrum) of the display this effect will be visible as higher noise power near to the nyquist frequency of the display. For some applications like medical imaging this higher noise power is unacceptable.